Cell growth and survival can be regulated by cytokine-induced signals. Nevertheless, the molecular mechanisms of signal transduction from cytokine receptors to mediators of cell growth and apoptosis are not well understood. STAT (signal transducer and activator of transcription) proteins can be activated by protein tyrosine kinases (PTK) in response to a variety of cytokines. They have shown that STAT activation is correlated with cell growth inhibition in response to EGF and IFN-g. Activated STAT proteins can regulate expression of some cyclin-dependent kinase (CDK) inhibitors, such as p21WAF1/CIP1, and induce expression of the ICE gene, which is involved in induction of apoptosis. Using a cell line which is deficient in STAT1, and a derived cell line into which STAT1a has been re-introduced, they demonstrate that STAT1 protein can cause cell growth suppression and apoptosis in response to IFN-g and EGF. On the basis of these results, Dr. Fu proposes a hypothesis that PTK-STAT signaling pathways can negatively regulate the cell growth and survival in response to cytokines. To further investigate functions of STAT proteins in cell growth control and apoptosis, they will do the following: 1) To create STAT deficient (knock-out) mice to investigate functions of STAT proteins in control of cell growth and survival in vivo. In particular, they will generate conditional and tissue specific STAT3 (-/-) mice using the LoxP-Cre system. 2) To utilize STAT3 knock-out mice, or cell lines derived from these mice, for analysis of functions of STAT3 in cell growth and apoptosis in response to several cytokines, including the IL-6 family of cytokines and EGF, PDGF, etc. 3) To analyze STAT6 functions in response to IL-4 using STAT6 (-/-) mice and derived cells. In particular, they will determine whether different forms of STAT6 have specific functions in growth inhibition or stimulation. 4) To analyze the functional roles of the STATs in cell cycle control, and possible cross-talks of the STAT pathway with the Ras-MAP kinase pathway and other mitogenic pathways during cell proliferation or apoptosis. Whether activation of STAT proteins exerts a general growth inhibitory effect on cell out-growth will also be determined. 5) To study the signaling pathways from PTK-STAT to induction of apoptosis in response to cytokines. Additionally, to study possible involvement of PTK-STAT pathways in cancer development and treatment. Possible STAT activation in other abnormal PTK activation systems will also be investigated. Dr. Fu believes that the experiments proposed in this proposal will explore a new field of STAT functions in cell growth control, apoptosis, cancer development, and will contribute to further understanding of the signal transduction mechanisms controlling both positive and negative regulations of a cell.